38 CORNISH POST-TERTIARY GEOLOGY. 



The small flint and quai'tz pebbles noticed in soil on the top of 

 the clitfs near Trewavas Head, might likewise be considered as 

 of Tertiary origin, 



Mr. A. Smith ' mentions the occurrence of small chalk flints not 

 much worn, and of fragmentary stones of Greensand on the surface 

 of Castle Down, on the north part of Tresco Island. 



Mr. Peach ^ speaks of the " Chalk of No Best " off the Dodman 

 Point. Both these observations require confirmation, especially the 

 latter, so I give them without further comment. 



The sands and clays of St. Agnes, occurring at a lieight of from 

 350 to 400 feet above the sea, and occupying a site quite disconnected 

 with the present drainage system, carry us back to a time so far 

 removed from the present, that subsequent agencies have obliterated 

 the relations of their site to its original surroundings, in the elabo- 

 ration of a new drainage system. If, as De la Beche suggested, 

 they are of marine origin, deposited, as Messrs. Kitto and Davies 

 think, in " a sheltered arm of the sea, into which a river emptied its 

 waters," the land must have been submerged to a depth of 400 feet, 

 and therefore their origin must be put back to a time very much 

 more remote than that during which the Eaised Beaches were 

 formed. 



The exceptional development of clay where the deposit rests on 

 granite, and the peculiarly local character of the pebbles and 

 boulders met with in the different pits, seems to me to forbid a 

 marine origin. The thinning out of the clay toward the edges of 

 the main deposit, as shown in Section F and Pit No. 2, and in Section 

 B in Messrs. Kitto and Davies's paper ; the impersistent character 

 of the beds, as shown in the three sections of Pit No. 1 ; the absence 

 of all organic remains, save a very doubtful plant-like marking, are 

 much more easily explainable by fluviatile than marine action. The 

 exposure in 1875 of a cliff face (part of which is shown in Section 

 ¥) 16 feet high, at 15 feet from the surface, with very large pebbles, 

 l)oulders, and angular fragments near its base, corroborates the 

 information De la Beche obtained, and might be taken as proving 

 marine action ; but, on the other hand, the very great irregularity 

 in the shelf in some stream-tin sections, the existence of false shelf 



' Trans. Roy. Geol. Soc. Com. vol. vii. p. 313. 

 * Ibid. vol. v. p. 55. 



